The present invention relates to a screw pump having a pair of intermeshing screw rotors.
As a conventional screw pump, a screw fluid machine is disclosed by Japanese Patent Application Publication No. 2001-182679 or No. 2001-193677. This type of screw pump includes a pair of intermeshing screw rotors and a housing for accommodating the rotors. The housing has an inlet port at one end thereof for allowing fluid to be drawn therethrough into the housing, and an outlet port at the other end thereof for allowing the fluid to be discharged therethrough out of the housing. Each rotor is of a single-start thread and its lead angle decreases steplessly at a constant rate as the thread of the rotor approaches the outlet port thereby to form a changing lead portion of the rotor. It is noted that the lead angle is an angle made between a plane that is perpendicular to the axis of the rotor and the helix of the thread of the rotor. As the screw pump is operated, the fluid approaches the outlet port while decreasing its volume.
The same kind of technique is disclosed by Japanese Patent Application Publication No. 2001-55992 or No. 11-270485. Japanese Patent Application Publication No. 2001-55992 discloses a displacement machine for compressible medium, and its rotors are of a multi-start thread. Each rotor includes a changing lead portion that decreases steplessly at a constant rate as the thread of the rotor approaches the outlet port, and a constant lead portion whose lead angle is constant. Japanese Patent Application Publication No. 11-270485 discloses a vacuum pump including a pair of rotors each having a changing lead portion and a constant lead portion.
Meanwhile as the space closed by a pair of rotors and the housing at the time of one rotation of the rotors, or one turn, is large, working efficiency of drawing the fluid into the screw pump improves. The space is hereinafter referred to as a pump space. In the above techniques disclosed by the four references, however, there is not disclosed concrete structure for positively increasing the volume of the closed pump space formed by the changing lead portion. That is, in each of the conventional screw pumps, the volume of the pump space formed by the changing lead portion is not necessarily set as a suitable volume for improving the efficiency.
The present invention is directed to a screw pump having a pair of screw rotors with a changing lead portion whose lead angle changes, wherein volume of a pump space of the screw pump which is closed at the time of one turn of the rotors is increased compared to that of the conventional screw pump thereby to improve efficiency of drawing fluid into the screw pump.